Data-Over-Cable Service Interface Specifications (DOCSIS®) are standards specifying protocols for transmitting signals over cable networks (e.g., cable-television (CATV) networks). DOCSIS ensures interoperability between cable operators' equipment and cable modems at subscriber locations. The first DOCSIS specification, DOCSIS 1.0, was designed to enable multi-vendor interoperability. DOCSIS 1.0 supports best-effort transmissions but does not offer the capability to guarantee performance of a given service. The following DOCSIS specification, DOCSIS 1.1, provided quality-of-service (QoS) features that went beyond best-effort services by enabling delivery of services that require more than best-effort transmission capabilities. DOCSIS 1.1 is fully backwards-compatible with DOCSIS 1.0, which allows DOCSIS 1.1 cable modems to coexist with already-deployed DOCSIS 1.1 equipment.
Recently, DOCSIS 2.0 has been approved and adopted by organizations that administer telecommunications standards (e.g., the International Telecommunications Union (ITU)). DOCSIS 2.0 certified equipment has been and is now being deployed in cable networks. DOCSIS 2.0 specifies several features that make it appealing over the earlier DOCSIS standards, including improved performance, increased throughput, and more efficient use of network capacity. For example, DOCSIS 2.0 specifies improved protection from impairments on CATV networks, increased support for transmit pre-equalization techniques, and advanced physical layer modulation techniques (Advanced Time Division Multiple Access (ATDMA) and Synchronous Code Division Multiple Access (SCDMA)) that support higher modulation levels. A primary advantage of DOCSIS 2.0 is improved upstream performance. For example, DOCSIS 2.0 can triple the maximum upstream capacity offered by DOCSIS 1.1. DOCSIS 2.0 enables transmission across a 6.4 MegaHertz (MHz) channel and increases upstream performance to levels reaching bandwidths of 30.72 Megabits per second (Mbps) using 64 quadrature amplitude modulation (QAM) or 128 QAM and Trellis Coded Modulation (TCM).
While DOCSIS 2.0 provides improved performance over previous DOCSIS standards, migration from the previous DOCSIS standards to DOCSIS 2.0 is not free of technical and economic challenges. One significant concern is the effects that DOCSIS 2.0 equipment will have on the performance of already-deployed DOCSIS 1.0 and DOCSIS 1.1 (collectively “DOCSIS 1.x”) equipment. Although DOCSIS 2.0 specifies backwards-compatibility with DOCSIS 1.x equipment for mixed mode operation, the DOCSIS 1.x equipment does not support DOCSIS 2.0. This results in additional overhead that will cause DOCSIS 1.x cable modems to experience degraded performance as DOCSIS 2.0 equipment is deployed.
Another significant concern results from the higher modulation rates supported by DOCSIS 2.0. Higher modulation rates require higher signal-to-noise ratios from the cable network. Standard DOCSIS 2.0 network equipment provides for the ability to measure signal-to-noise ratios (SNR) that would be required to transition to higher modulation rates specified in DOCSIS 2.0 (e.g., 64-QAM). However, the measurement techniques provided for in DOCSIS 2.0 are inherently flawed for higher modulation rates. For example, DOCSIS 2.0-specified measurement techniques cannot afford the time required to accurately measure channel performance for higher modulation rates because the conventional measurement techniques adversely affect performance levels on channels being measured. In other words, quality-of-service (QoS) and measurement accuracy are at odds with each other. If measurement accuracy is increased, QoS suffers, which can cause service disruptions, performance below guaranteed QoS, and customer dissatisfaction. Thus, measurement accuracy must be kept to a minimum to maintain QoS in conventional networks. Significantly, conventional DOCSIS 2.0 measurement techniques cannot afford to take the time required to obtain precise enough measurements that accurately predict performance and SNR margins at higher modulation rates. Because of this shortcoming, DOCSIS 2.0 specifies “fall back” procedures that return from a high modulation rate to a lower modulation rate when the high modulation rate fails due to inaccurate measurements.
To make matters worse, DOCSIS 2.0 measurement sequences are commonly performed using the active receiver and its configuration which may likely be quadrature phase shift keying (QPSK) (also referred to as 4-QAM). While QPSK measurements are sufficient to measure linear impairments, QPSK measurements cannot accurately estimate non-linearity in cable networks. This is fine for operating in QPSK, which is somewhat immune to network non-linearity. However, network non-linearity can preclude successful operation at higher modulation rates (e.g., 64-QAM and higher), which are much more susceptible to network non-linearity. As a result, conventional cable networks are not aware of and cannot operate at higher modulation rates in the presence of network non-linearity. Modulation rates are often forced to “fall back” to lower modulation rates because DOCSIS 2.0's limited QPSK measurement techniques cannot take the time to accurately measure upstream performance or to account for network non-linearity. Therefore, conventional DOCSIS 2.0 equipment does not fully utilize available bandwidth on upstream channels.